1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an image forming apparatus with an intermediate transfer system that transfers a toner image carried on an intermediate transfer belt to a sheet.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, followed by heating and pressurization for fixing, whereby an image is formed on the sheet.
In recent years, image forming apparatuses are required to form high-quality images on a wide variety of sheets. In response to such a need, an intermediate transfer system using an intermediate transferrer such as an intermediate transfer belt has become mainstream. In the intermediate transfer system, respective toner images of yellow (Y), magenta (M), cyan (C), and black (K) formed on photoconductor drums are transferred (primarily transferred) to the intermediate transferrer, and the toner images of the four colors are superimposed on each other on the intermediate transferrer. Then, the resultant image is transferred (secondarily transferred) to a sheet.
FIG. 1 illustrates a configuration in the vicinity of a secondary transfer section that transfers a toner image carried on an intermediate transfer belt to a sheet.
As illustrated in FIG. 1, intermediate transfer belt 421 is stretched on a plurality of support rollers 423 including backup roller 423A. Secondary transfer roller 431A is brought into pressurized contact with backup roller 423A, whereby transfer nip N is formed. Sheet S is caused to pass through transfer nip N, whereby a toner image carried on intermediate transfer belt 421 is transferred to sheet S.
In general, intermediate transfer belt 421 enters transfer nip N so as to be inclined to the line connecting between the shaft centers of backup roller 423A and secondary transfer roller 431A. Upstream of transfer nip N in the sheet conveying direction, sheet S is pressed against intermediate transfer belt 421, to be thereby conveyed while being curved along secondary transfer roller 431A. Sheet S is thus nipped (so-called pre-nipped) also upstream of transfer nip N in the sheet conveying direction, and hence the transfer performance is improved.
When such an image forming apparatus as described above forms an image on sheet S having a high stiffness, such as cardboard, electric discharge occurs in the vicinity of the entrance of transfer nip N, and erroneous transfer of transfer dust and the like may occur. Specifically, when the rear end of sheet S enters transfer nip N, the rear end of sheet S pushes up intermediate transfer belt 421 (so-called rear-end flick occurs), whereby a gap is formed between sheet S and intermediate transfer belt 421. The gap thus formed causes electric discharge (see FIG. 2).
For example, Japanese Patent Application Laid-Open No. 2010-139603 discloses the following technique for preventing electric discharge in the vicinity of the entrance of a transfer nip. That is, at the timing at which the rear end of a sheet comes out of a guide member, an intermediate transfer belt is displaced upstream of transfer nip N in the sheet conveying direction, whereby the impact when the sheet collides against the intermediate transfer belt is reduced.
Japanese Patent Application Laid-Open No. 2010-2838 discloses that a sheet is conveyed substantially perpendicularly to the line connecting between the shaft center of a secondary transfer roller and the shaft center of a counter roller, and an electrode member is placed inside of an intermediate transfer belt, whereby an electric field that is formed in a gap in the same direction as that of a transfer electric field is weakened.
Unfortunately, according to the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2010-139603, although the impact when the sheet collides against the intermediate transfer belt is reduced, the intermediate transfer belt bends inwardly when the rear end of the sheet enters the transfer nip, so that a non-negligible gap is formed. Hence, the occurrence of electric discharge cannot be reliably prevented. Further, if the intermediate transfer belt is displaced, the nip width between the intermediate transfer belt and a secondary transfer roller unfavorably changes between before and after the displacement. Hence, color deviation, tone deviation, and the like may occur in a formed image.
The image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2010-2838 has a configuration in which the sheet is conveyed substantially perpendicularly to the line connecting between the shaft center of the secondary transfer roller and the shaft center of the counter roller, that is, a configuration in which a rear-end flick of sheet on the intermediate transfer belt does not occur. Hence, Japanese Patent Application Laid-Open No. 2010-2838 does not refer to a gap formed by the rear-end flick.